staging: comedi: s526: tidy up s526_ai_rinsn()

Rename this function to follow the normal naming in comedi drivers.

Use the comedi_offset_munge() helper to munge the hardware two's
complement data to the comedi offset binary format.

Change the final return to insn->n to clarify the return value without
the need for the comment.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>